feat(settings): configurable provider retry ceiling and first-retry wait

[shabhui]

Summary

Adds two optional numeric settings that customize the shared provider retry machinery (429 rate-limit and transient-failure budgets) without removing its guard rails:

• providerRetryMaxAttempts — any non-negative integer ceiling shared by both budgets. 0 means unlimited, exactly like the existing infiniteProviderRetry switch; the switch keeps winning over any configured value.
• **providerRetryInitialDelayMs** — any non-negative first-retry wait in milliseconds. 0 retries immediately. Later retries keep doubling from it, and the exponential cap rises with it (max(shipped cap, configured wait)), so a long custom interval is honored rather than truncated. Server Retry-After` keeps precedence in both backoffs.

Design notes

• Ranges are deliberately loose: the UI and both write boundaries only reject NaN / negative / fractional junk. No artificial upper bound on either field.
• Both fields normalize on read and validate on write at renderer and main boundaries; stored junk drops back to the shipped defaults (10 retries, 2000 ms).
• The retrying activity now carries the effective ceiling (maxAttempts), so the transcript status shows attempt 3/7 instead of the hardcoded /10; unlimited shows ∞.
• Turning infiniteProviderRetry back off restores the configured ceiling (the switch no longer pins the internal budget to 0), and live session reuse updates the policy through the new setProviderRetryPolicy.
• The stream-retry adapter receives the custom pacing through an optional initialDelayMs controller hook, so pre-stream and mid-stream retries share one pacing.

Files

• packages/shared: retry settings type + normalize helpers + retrying-activity maxAttempts field.
• packages/agent-runtime: budget checks honor the ceiling (0 = unlimited); both delay functions accept initialDelayMs and raise their caps; sidecar reuses/constructs with the policy.
• apps/desktop: settings UI rows (General, next to the infinite-retry switch), write validation on both boundaries, transcript retry status, settings search keys.
• packages/i18n: 9 locales.
• Spec docs (03-runtime/02-agent-runtime.md) updated in en + zh-CN.

Test plan

• packages/shared vitest: 1176 passed (4 new normalize cases)
• packages/agent-runtime provider-retry.test.ts: 48 passed (5 new custom-pacing cases: scaling, cap raising, Retry-After precedence, zero-immediate, jitter)
• packages/agent-runtime runtime.test.ts: 276 passed (new: smaller custom ceiling across both budgets, zero-ceiling unlimited, toggle-off restores ceiling, retrying activity reports maxAttempts)
• apps/desktop settings-roundtrip.test.mjs: 5 passed (round-trip of both fields incl. 0/0, junk rejection at both boundaries, renderer drops stored junk)
• tsc -p tsconfig.json --noEmit clean for agent-runtime and desktop
• biome check on all touched files
• CI full suite

Closes #759 follow-up territory: gives finite custom ceilings and pacing instead of only the unlimited boolean.