fix(task-board): name lanes after the board's columns, not Studio's

[viktormarinho]

Follow-up to the mirrored-board series. On a board whose columns come from a
tracker, three surfaces were still speaking Studio's vocabulary. Each one
misled differently.

A stranded card was masquerading as a column

The board draws a lane for any status no column accounts for (#6727), so a card
can't go invisible. But that lane was labelled with our translation — a
triage card on a mirrored board rendered as "Backlog", sitting next to the
tracker's own Backlog column. Two identical-looking columns, one of which does
not exist over there and nobody can act on.

laneHeader(status, t, columns) now answers naming and appearance together,
because they're the same question: a column the board owns is drawn with its
own name and, if it's one of ours, our icon. A status off the board gets
neither — it's labelled by its raw key and carries an off board badge with a
tooltip saying why it's on screen. The "new task here" button is gone for it
too: a card created in a non-column would be stranded the moment it existed.

laneLabel is now module-private. Naming a lane requires knowing whether the
board even has a column for it, and every caller that skipped that question
is the bug above.

You couldn't drag a card into an empty column

A lane's droppable was validated against Studio's STATUSES, which on a
mirrored board matches no column at all — so dropping onto an empty column
resolved to null and the card sprang back. Only dropping onto an existing card
worked, which made it look like an intermittent bug rather than a rule.

Extracted as dropLane, validated against the board's columns, with the two
ways of being over a lane (its droppable, or a card in it) gated by one rule.
That closes the mirror-image hole too: hovering a card in an off-board lane
used to resolve to that lane, so the drop appeared to work until the list
refetched and the server's rejection showed.

"Move to" was a list of ways to fail

The bulk menu and the task's status dropdown offered the canonical lanes
whoever's board it was. On a mirrored board every entry named a column the
server rejects. moveTargets takes the board's columns now; useBoardColumns
reads them off the same cached list without opening the board's SSE streams.

STATUSES is deleted — it had become a second copy of CANONICAL_COLUMN_KEYS
with no reader but a test.

Verification

Against a local board with five mirrored columns and one Studio-native card
stranded off it:

• Lanes render the tracker's names; the stranded card's lane reads its raw
  status with the off board badge and no new-task button.
• The status dropdown lists exactly the five columns — no canonical lanes.
• A card dragged into a previously empty mirrored column landed and
  persisted (verified in Postgres). That run predates the dropLane
  extraction; the extraction itself is covered by unit tests, and the wiring
  is four lines.

Unit tests cover dropLane (4), laneHeader (3), moveTargets on a mirrored
board, and laneVisibility's new unplaced. bun run check, lint, fmt,
and knip are clean.

Still open

• A renamed tracker column still reads as delete + create — the Agile API gives
  no stable column id, so the key is the name.
• Bulk/import writers still leave board_column_org null.
• in_review roles are settable but read by nothing yet.

---

Summary by cubic

Fixes the task board so lanes, drag targets, and the move menu follow the board's own columns instead of Studio's canonical statuses. On boards mirrored from a tracker, canonical names matched no real column, so stranded cards looked like phantom columns, drops into empty columns failed, and "Move to" listed statuses the server rejects.

Bug Fixes

• A lane with no matching column now shows its raw status key with an "off board" badge and no "new task here" button.
• Drops are validated against the board's columns, so mirrored empty columns accept cards and off-board lanes refuse them.
• "Move to" and the status dropdown now list only the board's own columns.

Refactors

• laneHeader replaces the public laneLabel, which is now module-private; naming a lane requires knowing whether the board has a column for it.
• dropLane handles both ways of being over a lane — its droppable or a card in it — with one rule.
• STATUSES is deleted; moveTargets and useBoardColumns read columns from the same cached query as the board.

^{Written for commit 582599a. Summary will update on new commits.}