fix(lint): clear all 15 oxlint warnings

[aniruddhaadak80]

Summary

Clears all 15 oxlint warnings (npm run lint: 15 warnings / 0 errors before, 0 / 0 after). No behavior change — dead stores, useless fallbacks, and unused bindings only:

• web/static/js/diffPanel.mjs:41,59,82 — removes write-only #taskId (staleness is tracked by #requestId; the field was assigned in load/clear but never read).
• web/static/js/sessionPanel.mjs:28,134 — removes write-only #tab (#setTab assigned it but nothing reads it; the DOM active class is the source of truth).
• web/static/js/threadBoard.mjs:17 — drops the useless ?? {} spread fallback (spreading undefined is already a no-op).
• web/static/js/themeBoot.js:16, web/static/js/webToken.js (7 sites: lines 46, 55, 80, 267, 347, 416, 441) — bare catch where the binding was never used.
• src/support/web.tailscale.test.ts:12-13 — removes unused readFileSync / isTailscaleAddress imports (tests import the latter dynamically from ./web.js).
• src/adapters/webTools.test.ts:160 — ?.['X-VEGA-Key'] so the optional chain cannot throw when init is undefined.
• src/automation/dailyReporter.ts:26-33 — removes uncalled readWatermark (only writeWatermark is ever invoked) and its now-unused readFileSync import.

Related issue

No open issue exists (the repo currently has zero open issues). Proactive hygiene discovered by running the repo's own gate (npm run lint) on a clean checkout — CI runs lint first, so warning-free output keeps the gate signal clean.

Type of change

• Bug fix
• New feature
• Refactor / chore
• Docs

Checklist

• npm run lint passes (0 warnings, 0 errors; was 15/0)
• npm run typecheck passes
• npm run build passes — not run to completion locally (full tsc emit exceeds the local command timeout on this Windows checkout); typecheck (tsc --noEmit -p tsconfig.check.json) passes on the touched TS files
• npm test passes for the touched areas (tests/web/diffPanel.test.ts: 13 passed; tests/web/threadBoard.test.ts: 5 passed; src/adapters/webTools.test.ts: 23 passed; tests/web/sessionSelection.test.ts + tests/web/format.test.ts: 11 passed). Note: src/support/web.tailscale.test.ts (touched only by removing two provably-unused imports) cannot complete locally — it times out identically on clean main (importing ./web.js hangs on this Windows checkout), so this is pre-existing and unrelated
• Docs updated if behavior changed (no behavior change)
• Commits follow Conventional Commits