fix(plugins): refuse a concurrent install instead of reporting success

[kipavy]

Follow-up from the #150/#152 review: the only case in the deep-link confirm flow where the UI actively lies.

The bug

marketplaceStore.installPlugin returned silently when the requested id was already in installing:

if (installing.has(plugin.id)) return;

Every caller reads that resolve as a completed install. Accept a plugin-install deep-link sheet while the Settings tab is installing the same plugin and the sheet resolves, dismisses, and tells the user it worked — having written nothing.

The fix

The second request throws PluginInstallInProgressError (new, in src/plugins/installErrors.ts), which pluginInstallErrorMessage names for the user via a new settings.plugins.install.alreadyInstalling key in all four locales. Both the Settings toast and the confirm sheet already route through that helper, so this adds no second instanceof ladder.

Refused rather than joined to the in-flight install, deliberately. The sheet and the Settings tab can hold different reviewed manifest texts for the same id (reviewedManifestText is what closes the fetch→consent→load TOCTOU). Joining would install the other caller's reviewed bundle while telling this caller its consent was honoured — trading a false success for a consent bypass.

Callers after the change:

• deep-link confirm sheet — shows "already being installed", stays honest.
• PluginsSection — unreachable in practice (the button is disabled while busy), same toast path if it ever is.
• plugins.ts domain layer (MCP install_plugin) — propagates the message instead of the misleading produced no loaded plugin.
• restoreMissingPlugins — sequential and already catching; unaffected.

Tests

Three added to marketplaceStore.installUpdate.test.ts, sharing one gatedFetch() helper that parks the first install inside plugin_fetch_url:

• a second install of an id already installing is refused
• a refused concurrent install does not clear the running install's busy state
• an id is installable again once its previous install has settled

All three fail on dev, pass here.

Verification

• Full CI=true vitest run: 479/480 files, 3718/3719 tests. Sole failure is tests/pluginBundleBuild.test.ts (39237ms — the known contention flake); re-run alone with the edited file it is 17/17 green.
• Real typecheck node ./node_modules/typescript/bin/tsc --noEmit -p tsconfig.json: exit 0, no output.