Add a Blocked tile to the Overview

[barnaclebarnes]

Closes #9.

Jobs held by a concurrency limit were invisible on the Overview: the backlog read as ready + scheduled, so you had to open the Jobs list to find out how much work was actually left. Blocked jobs are work the application still owes, so they belong in the tile row.

Changes

• Flightdeck::Overview gains a blocked count (SolidQueue::BlockedExecution, through the same cached counted helper as every other tile) and a blocked_tile slotted between "Ready now" and "Scheduled". The detail line reads "held by concurrency limits", or "nothing blocked" at zero.
• The tile grid was hardcoded at six columns. It is now seven across on a wide screen and drops to four (a 4 + 3 row) below 1320px — measured in the browser, "PROCESSED · 24H" starts wrapping to two lines somewhere between 1280 and 1290px at seven columns, so 1320 leaves margin for the other configured UI fonts. The 640px → two-column rule is unchanged, and .fd-grid-2 keeps its own 1080px breakpoint.
• Rebuilt assets, regenerated docs/screenshots, changelog entry under [Unreleased].
• Tile-count assertions updated in overview_test.rb, nested_mount_test.rb and system/dashboard_test.rb, plus a new assertion that the tile reports the real blocked count.

Verification

• bin/test (both host modes): 277 runs, 0 failures
• bundle exec rake test:system: 13 runs, 0 failures
• bundle exec rubocop clean, bundle exec rake assets:check fresh

Not included

Queue depth on the Queues panel still counts ready executions only, so a queue with blocked work shows a smaller depth than the tile implies. Widening that changes latency semantics too, so it seemed like a separate call — happy to fold it in if you'd rather.

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