feat: switch active OpenCode accounts from all Tally surfaces

[MaxAnderson95]

Tally tracks multiple subscription accounts but does not show which account OpenCode has selected or let users change it. This adds active-account status and explicit switching to the native app, web UI, and OpenCode companion.

Changes

• Show "Active in OpenCode" and "Use in OpenCode" on account cards; expose active in account responses and an activate companion action.
• Route all switches through the shared owner and OpenCode's activation API. OpenCode performs the database write and emits the event that reloads provider state; Tally's database access stays read-only.
• Verify the configured database and stored account, serialize switches, and reread selection after activation. Hide unconfirmed selection and avoid automatic retries after response loss.
• Document the running-service requirement and explicit-user-request behavior for the companion.

Verification

• bash scripts/test-swift.sh: 92 tests passed, including selection ordering, provider isolation, concurrent switches, lost responses, and HTTP policy.
• npm --prefix web test: 20 tests passed; npm --prefix web run build passed.
• npm --prefix companion test: 16 tests passed; npm --prefix companion run build passed.
• bash scripts/build-app.sh and code-signature verification passed. Installed locally and verified that the running API reports active status for all seven stored accounts and serves the new web assets.
• Switching tests use synthetic accounts. Live switching and visual UI interactions remain unverified; browser tools were unavailable.

Originally created in OpenCode session ID: ses_f64815ab1ffexWF6igUlIRbp0j using model: openai/gpt-6-astra