fix(ipc): validate base64 format in audio:save schema (#250)

[tbrandenburg]

Issues fixed

Closes #250

Summary

audio:save's handler wrapped Buffer.from(base64Audio, "base64") in a try/catch, but Node's lenient base64 decoder never throws, so the catch was dead code. Malformed base64 payloads silently wrote a garbage .webm file to disk instead of surfacing an error.

• Added a .refine() to saveRecordingSchema.base64Audio in src/shared/schemas.ts validating base64 charset/padding/length (^[A-Za-z0-9+/]*={0,2}$ + length % 4 === 0) before any decode/write happens.
• Removed the now-fully-dead try/catch around Buffer.from in src/main/ipc.ts's audio:save handler, since validation now happens at the schema boundary.
• Added a regression test in src/main/ipc.test.ts asserting malformed base64 is rejected with a clear error and no file is written to os.tmpdir().

Validation commands run

• npx vitest run src/main/ipc.test.ts src/shared/schemas.test.ts → 40/40 passed
• npx tsc --noEmit -p tsconfig.json && npx tsc --noEmit -p tsconfig.main.json → clean
• npx vitest run (full suite) → 422 passed, 15 pre-existing failures confined to src/main/model/* (model catalog/registry tests), confirmed to fail identically on main before this change (unrelated, likely network-dependent fixtures)

E2E coverage

No dedicated E2E test added; this is an IPC-boundary validation fix covered by the existing src/main/ipc.test.ts integration-style test (invokes the real IPC handler registry, not mocked), which is the appropriate test layer for this change per repo conventions (renderer only ever sends real base64 from MediaRecorder, so this path isn't reachable through the normal UI flow — see issue for severity rationale).

Risks / follow-ups

• Risk: low — additive validation only; well-formed base64 (the only input from the real renderer flow) is unaffected; existing tests for valid audio save and mimeType rejection still pass.
• Follow-up (not fixed here, out of scope): a pre-existing flaky test was observed once during subagent validation ("resolves a bundled extension's provider on the chat path... (issues #211, #212)") that passed on immediate re-run; not reproduced during coordinator's final full-suite run, so no action taken. The 15 src/main/model/* test failures are pre-existing on main and unrelated to this change; no new issue filed since they already fail on main without any modification.

[github-actions]

⚠️ Test duration budget warning

@tbrandenburg the test suite exceeded its duration budget:

• [SOFT] /home/runner/work/pi-desktop/pi-desktop/src/main/settings/store.test.ts (node) took 2.64s (threshold: 1.0s)