fix(capture): retry content-script message on one-click auto-capture

[DCCA]

Symptom

After enabling one-click capture (#8), clicking the toolbar icon opened the editor but capture failed with:

Could not establish connection. Receiving end does not exist.

The manual Capture Page button still worked.

Root cause

One-click auto-capture fires immediately on editor load. captureFullPage sends SB_GET_PAGE_METRICS to the target tab's content script — but that listener hasn't registered yet that early, so chrome.tabs.sendMessage rejects with the no-receiver error. ensureInjectable swallowed its own injection error and there was no retry, so capture aborted. The manual button "worked" only because the user clicks seconds later, by which point the script is ready.

This also affected tabs that were already open before the extension was (re)loaded — static content_scripts don't run on pre-existing tabs, so they relied entirely on the one-shot programmatic injection.

Fix

sendToContentScript(tabId, message, { retries, delayMs }): send the message; on a no-receiver error, re-inject content.js and retry with a short backoff (up to 6 times), rethrowing any other error immediately. Used for the racy metrics call. isNoReceiverError is extracted and pure.

Verification

• New unit tests (tests/capture.test.ts): first-try success, retry-then-succeed (asserts re-injection count), immediate rethrow of unrelated errors, give-up after exhausting retries, plus isNoReceiverError matching. 48 tests pass (was 42).
• npm run typecheck, npm run lint, npm run format:check, npm run build green.
• Confirmed the retry logic is present in the built dist/assets/editor.js.
• Live confirmation (icon → auto-capture succeeds) to be done by @DCCA after reload — no runner exists for the live extension.

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