refactor: dedup TranslateFn helpers and controllable-open into @multica/core

[devin-ai-integration]

Summary

jscpd flagged two patterns duplicated across the views package. Both are extracted into @multica/core and adopted at every call site — no behavior change.

1. The t translate-prop pattern (~27 files). Each view component redeclared the same type and fallback logic:

type TranslateFn = (key: string, fallback: string) => string;   // declared in 25+ files
const defaultT = (_key, fallback) => fallback;                  // in 12+ files
const translate = t || defaultT;                                // in 12+ files

Now provided once in packages/core/hooks/useTranslation.ts:

export type TranslateFn = (key: string, fallback: string) => string;
export const fallbackT: TranslateFn = (_key, fallback) => fallback;
export function withT(t?: TranslateFn): TranslateFn { return t ?? fallbackT; }

Call sites collapse to import { withT, fallbackT, type TranslateFn } from "@multica/core" and either const translate = withT(t) or a t = fallbackT default param.

2. Controlled/uncontrolled open state (3 pickers). assignee/priority/status pickers each had:

const [internalOpen, setInternalOpen] = useState(false);
const open = controlledOpen ?? internalOpen;
const setOpen = controlledOnOpenChange ?? setInternalOpen;

Replaced by a new useControllableOpen(controlledOpen, controlledOnOpenChange) hook in packages/core/hooks/use-controllable-open.ts returning [open, setOpen].

Notes

• useControllableOpen, withT, fallbackT and the TranslateFn type are re-exported from the @multica/core barrel.
• issues-page.test.tsx's @multica/core mock gains the new exports (withT, fallbackT, useControllableOpen) since the components under test now consume them.
• Verified: pnpm typecheck clean on changed files, pnpm test for core passes, views test suite unchanged vs. base (the 7 issue-detail.test failures — api.getIssueWorkdirFiles is not a function — and the captcha typecheck/lint errors are pre-existing on main).

Link to Devin session: https://app.devin.ai/sessions/036672cd408042b191cb2beeb5405f75
Requested by: @killerleon94

[devin-ai-integration]

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