Add unit coverage for the web half of the #844 media consolidation

[selfcontained]

Summary

• Adds apps/web/src/lib/media-upload.test.ts (12 tests): isAcceptedUploadFile extension pre-filter — every accepted extension, case-insensitivity, multi-dot names, no-extension/trailing-dot/unsupported rejects, plus a parity test pinning the client pre-filter to the server's authoritative isMediaFile; uploadAgentMedia — asserts the real FormData payload (endpoint URL, POST method, file bytes + name, source default and override, inject flag presence semantics) and error propagation.
• Adds apps/web/src/components/app/media-file-utils.test.ts (5 tests): stripTimestamp suffix and mid-name stripping, date-only and hyphen-less near-misses left intact.

These modules were the untested web half of the #844 media consolidation (terminal-uploads.test.ts mocks @/lib/media-upload entirely, so none of this logic was previously exercised). The server half is already covered by apps/server/test/shared-media.test.ts.

Validation

• 9-mutant battery (toLowerCase drop, slice-past-dot, source/inject mutations, endpoint/method/return mutations, regex hyphen/digit mutations): 9/9 killed.
• pnpm run check, full unit suites (server 2362, web 516, ext 60, scripts 4), pnpm run finalize:web, and E2E (174 passed / 12 skipped, twice) all green.
• One general review agent inspected the diff: 0 must-fix, 0 should-fix, 3 nits (1 applied — dropped a non-discriminating duplicate assertion).

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