feat(webui): add limit bars, forecasts, events, exports and URL state

The Usage tab shows a bar for each configured limit. It turns amber at
80% and red at 100%. The tab also shows rejected requests per day, a
projected month-end cost with budget headroom, and CSV export for users
and groups. Names link to Charts with the matching scope.

The Charts tab ranks models over the full 24h, 30d or 12mo window. A
provider filter is applied on the server, and the ranking can be
exported as CSV. A new Events tab lists recent failures, filtered by
kind and user. The selected tab, filters and scope live in the URL
hash. Polling stops while the page is hidden. Views show which replica
answered, and a banner lists configuration warnings.

CSV cells that start with a formula character are neutralised. Links
sit outside the accordion buttons, so both controls keep their
accessible names. admin_assets_gen.go is regenerated.

Claude-Session: https://claude.ai/code/session_01V5jYJNxpDFdJkyyExpADrv

webui/src/lib/forecast.spec.ts

@@ -0,0 +1,107 @@
+import { describe, expect, it } from 'vitest'
+
+import { headroom, MIN_PROJECTION_FRACTION, monthProgress, projectMonthEnd } from './forecast'
+
+describe('monthProgress', () => {
+  it('reads 0 at the first instant of a UTC month', () => {
+    expect(monthProgress(new Date(Date.UTC(2026, 2, 1, 0, 0, 0)))).toBe(0)
+  })
+
+  it('reads exactly 0.5 at the midpoint of a 30-day month (April)', () => {
+    expect(monthProgress(new Date(Date.UTC(2026, 3, 16, 0, 0, 0)))).toBe(0.5)
+  })
+
+  it('reads approaching 1 near the end of the month, never reaching or exceeding it', () => {
+    const p = monthProgress(new Date(Date.UTC(2026, 2, 31, 23, 59, 59)))
+    expect(p).toBeLessThan(1)
+    expect(p).toBeGreaterThan(0.99)
+  })
+
+  it('handles a leap February (2028 has 29 days) — Feb 15 sits near the midpoint of 29, not 28, days', () => {
+    const p = monthProgress(new Date(Date.UTC(2028, 1, 15, 12, 0, 0)))
+    // 14.5 days elapsed of 29 total = exactly 0.5.
+    expect(p).toBeCloseTo(0.5, 10)
+  })
+
+  it('handles December -> January rollover without a hardcoded month-length table', () => {
+    const p = monthProgress(new Date(Date.UTC(2026, 11, 16, 0, 0, 0)))
+    expect(p).toBeCloseTo(15 / 31, 10)
+  })
+})
+
+describe('projectMonthEnd', () => {
+  it('returns null below MIN_PROJECTION_FRACTION — too early in the month to extrapolate', () => {
+    expect(projectMonthEnd(1_000_000, MIN_PROJECTION_FRACTION - 0.001)).toBeNull()
+  })
+
+  it('returns null at elapsedFraction 0 (the very first instant) rather than dividing by zero', () => {
+    expect(projectMonthEnd(0, 0)).toBeNull()
+  })
+
+  it('returns null at exactly the threshold boundary minus epsilon, but projects at the threshold itself', () => {
+    expect(projectMonthEnd(1000, MIN_PROJECTION_FRACTION)).toBe(Math.round(1000 / MIN_PROJECTION_FRACTION))
+  })
+
+  it('linearly extrapolates mtd/elapsedFraction, rounded to the nearest whole micro-USD', () => {
+    expect(projectMonthEnd(15_000_000, 0.5)).toBe(30_000_000)
+  })
+
+  it('rounds a non-exact division to the nearest integer', () => {
+    expect(projectMonthEnd(1, 0.3)).toBe(Math.round(1 / 0.3))
+  })
+
+  it('projects a full-month reading (elapsedFraction 1) back to itself', () => {
+    expect(projectMonthEnd(7_500_000, 1)).toBe(7_500_000)
+  })
+})
+
+// End-to-end monthProgress -> projectMonthEnd against real UTC clock times,
+// not just a bare elapsedFraction — pins MIN_PROJECTION_FRACTION's real-
+// world meaning (0.02 of a 30-day month is 14.4 hours, NOT minutes) against
+// an actual point in a month.
+describe('monthProgress -> projectMonthEnd (real clock times)', () => {
+  it('the first hour of a 30-day month is well under MIN_PROJECTION_FRACTION — no projection yet', () => {
+    const oneHourIn = new Date(Date.UTC(2026, 3, 1, 1, 0, 0))
+    expect(projectMonthEnd(100_000, monthProgress(oneHourIn))).toBeNull()
+  })
+
+  it('13 hours into a 30-day month is still under the threshold', () => {
+    const thirteenHoursIn = new Date(Date.UTC(2026, 3, 1, 13, 0, 0))
+    expect(projectMonthEnd(100_000, monthProgress(thirteenHoursIn))).toBeNull()
+  })
+
+  it('16 hours into a 30-day month has cleared the threshold and projects', () => {
+    const sixteenHoursIn = new Date(Date.UTC(2026, 3, 1, 16, 0, 0))
+    expect(projectMonthEnd(100_000, monthProgress(sixteenHoursIn))).not.toBeNull()
+  })
+})
+
+describe('headroom', () => {
+  it('returns all-null, willExceed false when projectedMicros is null (too early to project)', () => {
+    expect(headroom(null, 100)).toEqual({ limitMicros: null, remainingMicros: null, willExceed: false })
+  })
+
+  it('returns all-null, willExceed false when limitUsd is undefined (unlimited)', () => {
+    expect(headroom(5_000_000, undefined)).toEqual({ limitMicros: null, remainingMicros: null, willExceed: false })
+  })
+
+  it('returns all-null, willExceed false when limitUsd is 0 (LimitsConfig "unlimited" convention)', () => {
+    expect(headroom(5_000_000, 0)).toEqual({ limitMicros: null, remainingMicros: null, willExceed: false })
+  })
+
+  it('returns all-null, willExceed false for a negative limitUsd (defensive)', () => {
+    expect(headroom(5_000_000, -10)).toEqual({ limitMicros: null, remainingMicros: null, willExceed: false })
+  })
+
+  it('reports positive remainingMicros and willExceed false when the projection stays under the limit', () => {
+    expect(headroom(4_000_000, 5)).toEqual({ limitMicros: 5_000_000, remainingMicros: 1_000_000, willExceed: false })
+  })
+
+  it('reports negative remainingMicros and willExceed true once the projection crosses the limit', () => {
+    expect(headroom(6_000_000, 5)).toEqual({ limitMicros: 5_000_000, remainingMicros: -1_000_000, willExceed: true })
+  })
+
+  it('treats an exact match (projection == limit) as not exceeding (remainingMicros 0)', () => {
+    expect(headroom(5_000_000, 5)).toEqual({ limitMicros: 5_000_000, remainingMicros: 0, willExceed: false })
+  })
+})