Add dynamic badge text to the miner extension's toolbar icon reflecting local cache state

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Context

apps/gittensory-miner-extension stores its ranked opportunities (rankedCandidates) in chrome.storage.local via options.js, and opportunity-badge.js already renders a badge on matching GitHub issue pages. But background.js and manifest.json's toolbar action never call chrome.action.setBadgeText, so a contributor has no way to tell whether the extension currently holds live ranked data without first opening a matching issue page. This is distinct from #4862 (which only adds missing icon image assets) - that's a static-asset gap, this is dynamic runtime state.

Dependencies

None - independently shippable. It reads the same chrome.storage.local schema options.js already writes today and does not require the sibling savedAt/last-synced candidate in this batch to land first, though it pairs naturally with it if both ship.

Requirements

1. In background.js, read rankedCandidates from chrome.storage.local on extension/service-worker startup, and subscribe to chrome.storage.onChanged so the badge updates live whenever the stored value changes.
2. Call chrome.action.setBadgeText({ text }) to show the count of currently ranked opportunities as a short string (e.g. "12"); clear it (text: "") only for the genuinely-empty-array case, not the never-populated case (see requirement 3).
3. Distinguish "cache never populated" (no rankedCandidates key has ever been written) from "cache populated but currently zero opportunities" - render a distinct indicator (e.g. a dash) for the former so "no data yet" is never visually confused with "zero found".
4. Set an appropriate chrome.action.setBadgeBackgroundColor for the "has data" vs. "empty/stale" states so the distinction is visible at a glance, not just in the text.
5. Verify manifest.json's existing action key already covers setBadgeText/setBadgeBackgroundColor (it should, as these are part of the action API surface) - do not request any new host permissions.
6. Do not change options.js's storage schema or opportunity-badge.js's own per-issue-page badge - this PR is scoped strictly to the toolbar icon, a separate UI surface.

Deliverables / Acceptance Criteria

• background.js reads rankedCandidates on startup and on chrome.storage.onChanged
• Toolbar badge text shows the ranked-opportunity count when data exists
• Toolbar badge shows a distinct "no data yet" indicator when the cache has never been populated, separate from a genuine zero-count
• Badge background color differs between "has data" and "empty/stale" states
• No new permissions added to manifest.json; existing action key confirmed sufficient
• options.js and opportunity-badge.js are unchanged

Test Coverage Requirements

This PR must ship with full test coverage for every changed line and branch - the repo's Codecov patch gate requires 99%+ coverage and the house standard is to aim for 100%, including both sides of every conditional/nullish-coalescing branch introduced. Add: (1) unit tests covering the new/changed logic's success and failure paths (badge text/color for populated, empty-array, and never-populated storage states, plus the onChanged listener firing correctly), (2) an invariant test for 'the toolbar badge never displays a numeric count when rankedCandidates has never been written - true no-data is never rendered as zero', and (3) no regression test is required since this is a net-new feature, not a bug fix.

Codecov visibility note: packages/gittensory-miner/**, apps/gittensory-miner-ui/**, and apps/gittensory-miner-extension/** currently sit entirely outside vitest's coverage.include glob, so codecov/patch cannot measure changes there yet — closing that gap is exactly what #4864 and #4865 (in this same milestone) do. Any part of this change under packages/gittensory-engine/src/** or the repo's own src/** remains fully Codecov-instrumented as usual. Either way, this does not lower the bar: treat the 100%-including-invariants-and-regression target above as the enforced house standard regardless of what Codecov can currently see, and it becomes gate-enforced for real once #4864/#4865 ship.

Expected Outcome

A contributor can glance at the browser toolbar icon and immediately know whether the extension currently has fresh ranked-opportunity data, without first opening a matching GitHub issue page to check.

Links & Resources

• apps/gittensory-miner-extension/background.js
• apps/gittensory-miner-extension/manifest.json
• apps/gittensory-miner-extension/options.js
• apps/gittensory-miner-extension/opportunity-badge.js
• See Add icons to the extension manifest #4862
• Theme: Miner-ui/extension polish beyond Wave 5

[linear-code]

JSO-3730