feat(linear): functional Filter button + fix PATCH /api/issues 404

[obvious-autobuild]

Why

Two issues in the linear app, both surfaced while making the Filter control work:

1. The Filter button did nothing. In main-view.tsx the button rendered the Filter icon + label with no onClick, no state, and no UI — users could not narrow the issue list at all.
2. PATCH /api/issues/:id intermittently 404'd, so status changes never stuck. The backend kept issues in a module-level array. Netlify scales functions out to multiple isolated instances, so an issue created (or deleted) on one instance was invisible to a request routed to another. Reproduced on the preview deploy: a freshly created issue 404'd on ~70% of concurrent PATCHes, while seeded issues (present in every instance's initial array) always succeeded.

The avatar-contrast fix this PR originally also carried has since landed on main independently (getContrastTextColor in @/lib/utils). After rebasing, this PR drops its duplicate helper and reuses main's — and extends it to the one new avatar call site the Filter feature introduces (the assignee rows in the filter popover).

What

Filter feature — A Radix popover anchored to the Filter button lets users filter by status, priority, and assignee. Selections compose on top of the existing sidebar view filter (view filter first, then user filters); empty in a dimension means "no constraint". A count badge shows active filters, a clear action resets them, and a filter-aware empty state renders when nothing matches. New filtering logic lives in lib/issue-filters.ts (applyIssueFilters, countActiveFilters); filter state follows the existing Zustand ui-store pattern. The new assignee-row avatars use main's getContrastTextColor so initials stay AA-readable.

PATCH 404 (root cause) — Backed the issue collection with Netlify Blobs (getDeployStore, strong consistency), seeded from the static seed data, so every function instance shares one durable view. A local in-memory fallback keeps netlify dev working when Blobs is unavailable. The route regex now tolerates an optional trailing slash. Create/list/search/delete all go through the shared store, so no route can desync.

How to Review

• Filter: components/issue-filter-popover.tsx, components/main-view.tsx, lib/issue-filters.ts, store/ui-store.ts.
• PATCH fix: netlify/functions/lib/issue-store.ts (new durable store), netlify/functions/api.mts (routes now async + trailing-slash tolerant), netlify/functions/lib/data.ts (seed split into SEED_ISSUES/SEED_COUNTERS, makeIssue now pure).
• Rebased onto latest main; the avatar-contrast work is now main's getContrastTextColor, reused (not re-implemented) at the new filter-popover call site.
• Excluded: the dead "Display" button is intentionally left out of scope.

Test Evidence

• Filter / build: vite build succeeds (no runtime errors). Lint clean for all changed/new files (3 remaining errors are pre-existing in untouched files — confirmed on base main).
• PATCH root cause, reproduced on the preview deploy: 30 concurrent PATCHes on a freshly-created issue returned a mix of 200/404 (~70% 404); 30 concurrent PATCHes on a seeded issue all returned 200. After the Blobs-backed store, creates persist across instances.
• npm ci --dry-run passes with the updated package-lock.json (adds @netlify/blobs).

Acceptance Criteria

• Filter button opens a popover; filter by status/priority/assignee, combined with the view filter
• Filter dimensions combine (AND across dimensions; empty = no constraint)
• Active-filter count badge + clear action
• Filter-aware empty state when zero matches
• PATCH /api/issues/:id succeeds and persists status + patchable fields across refetch; no regression to list/create/delete
• Avatar initials meet AA contrast (via main's shared getContrastTextColor) at all call sites including the new filter popover
• Lint passes; Vite build passes

[github-actions]

Netlify PR Previews

Base: origin/main
Head: HEAD
PR: #10

Project	URL	Deploy
linear	replay-linear	logs

Replay QA

Project	Preview	QA Project	Result	Open Bugs
linear	preview	project	timed out	0

[obvious-autobuild]

Replying to this comment

Both open bugs from this QA run are addressed in the latest push (e7b7019):

• PATCH /api/issues/:id 404 (high) — root cause was the backend keeping issues in a module-level array that is not shared across Netlify function instances; a created/deleted issue on one instance was invisible to the next, so PATCH intermittently 404'd. Now backed by Netlify Blobs (getDeployStore, strong consistency) so every instance shares one durable view; route also tolerates an optional trailing slash. Reproduced the failure on the preview (freshly-created issue 404'd ~70% under concurrent PATCH) and confirmed seeded issues always 200.
• Avatar initials contrast (medium) — fallbacks hardcoded text-white; added a readableTextColor() luminance helper + shared InitialsAvatar. Verified: indigo 4.70:1, teal 8.58:1, orange 9.43:1 — all >= 4.5:1 AA.

A fresh QA run should pick up these fixes.