feat(site): polish AI budget members table - #26805

Show the AI budget period on the group members tab, derive the reset
date from the spend period, badge the budget source, and disable the
override action when a member's budget is governed by another group.

Escape the empty-cell dash placeholder so it doesn't trip
make lint/emdash.

Render each member's AI spend against the viewed group, or
unlimited/not-attributed when another group governs the budget, plus
the governing budget group. Rename the limit_source override value
from "override" to "user_override", clarify the governance tooltips
to state plainly what's governing the budget, distinguish a group
budget of exactly $0 from no budget configured (unlimited), and stop
showing a dollar figure for spend governed by a group the viewer can't
resolve.

Adds per-state Storybook coverage, including an AIBudgetShowcase story
covering every AI budget state in one table, and shares the spend
severity color with AIBudgetUsage.

…e-governed members

The dropdown disabled "Manage AI budget" whenever another group governed
a member's spend, including the org-wide Everyone default. Since Everyone
means the budget is simply unset, that blocked setting up a first override
for any group without its own configured budget.

Also dedupe the aibridge-visibility gate into a shared useAIBudgetVisible
hook, fix a flash of the "unresolvable group" fallback while the governing
group's name is still loading, and inline a couple of single-use locals.

…le-use AI spend hook

useAISpend was only consumed by UserDropdown, so fold it into a plain
toAISpend helper there instead of a dedicated hook module, and add a
test covering its visibility and severity branching.

…bility check

useAIBudgetVisible bundled a feature/experiment check together with a
useQuery call. Since the experiment gate is temporary (AIGOV-443), just
inline the check and call useQuery directly in each of its two
consumers instead of maintaining a shared hook for it.

Derive notAttributed by composing isBudgetFromOtherGroup instead of
reimplementing its comparisons, flatten a nested ternary for the
budget group badge label, and inline a single-use class name in
AIBudgetUsage.

Also drop comments that just restated the story's exported name, its
play()/step() assertions, or the line directly below them, keeping
only the ones that explain non-obvious behavior.