Rebuild the node cards on the app's own controls

[dovvnloading]

Problem

The Qt-to-React refactor modernized the app chrome and left the canvas node views behind. A QA pass over the seven plugin cards - read statically, then measured in the running app at 1440x900 with a fixture holding one node of every kind - found them still rendering controls the rest of the app had already replaced, and two cards unable to size or bound their own content.

• Gitlink's Context scope was the last native <select> in the app. CustomSelect backs every dropdown in Settings, Builder, Chat Library and the View popover, and no node view at all.
• Raw OS checkboxes in Web Research's retain-to-knowledge opt-in and Gitlink's per-file rows, while the View panel already ships a drawn checkbox introduced when the team replaced "the last engine-drawn controls" there. Most visible in the file list, where the control repeats once per row.
• Artifact's and Conversation's instruction inputs were rows={1} textareas with no height in CSS at all - they stayed one line regardless of how much was typed.
• Artifact bounded its card body and left the document preview unbounded, so a long draft pushed the turn history and the instruction row out of the visible card.
• Gitlink's Setup tab scrolled its file list inside an already-scrolling body - measured bodyClipped: true with a nested inner scroller, the same defect the code-sandbox card had before its rebuild: a max-height flex column shrinks its children below their content size unless told not to.
• Artifact's submit stayed on "Generate" while merely disabled during a request, where the sandbox card states "Running…".
• Gitlink labelled two of its four Setup fields and left the other two placeholder-only, and offered four equal-weight buttons with no primary.
• Web Research rendered a query box above blank space with nothing describing what a run produces.
• The HTML card shipped a permanently disabled "Popout" button explaining itself in a tooltip.

Change

Frontend and CSS only - no backend, no wire change, no contract regeneration.

• Gitlink's Context scope uses CustomSelect. It portals to the body and positions from getBoundingClientRect, so React Flow's viewport transform does not affect it.
• The View panel's drawn checkbox is extracted to a shared .gl-checkbox class and adopted by both node-view checkboxes; the View panel itself renders identically.
• Both instruction inputs auto-size natively (field-sizing: content with real bounds), the posture .composer-input and .code-sandbox-node-input already take.
• Artifact's document preview and turn history each scroll themselves, children no longer shrink below their content size, and the instruction row can never be scrolled out of reach.
• Gitlink's Setup tab keeps its children at content size and gets a larger budget, so the nested scrollbars collapse to a single scroll region.
• Artifact's submit reads "Generating…" / "Refining…" while in flight.
• All four Gitlink Setup fields are labelled, and Load Repo Tree is the tab's one primary.
• Web Research renders an empty state before its first run, in .library-empty-state's idiom at card scale.
• The Popout button is removed; the reason popout is unbuilt moves into the module doc, and a test asserts the control's absence so a dead placeholder cannot return.

Test plan

• npm run check (schema drift, typecheck, lint, vitest, build, bundle size) against a clean checkout of this branch: all green.
• New/updated tests: Gitlink renders no native <select> and exposes the Context scope trigger; its file-row checkboxes carry .gl-checkbox; the scope-mode test drives the real dropdown through the same trigger-then-option idiom SettingsDialog.test.tsx uses; Web Research shows the empty state initially, hides it beside a failure banner, and carries the drawn checkbox; Artifact asserts both the busy label and the disabled state, for the Generate and Refine cases; the HTML card asserts Popout's absence and that no window is opened.
• Measured live against the built bundle at 1440x900 with all nine node kinds on the canvas: zero native selects and zero unstyled checkboxes across every card; Artifact's body no longer clips, with its document preview as the single intended scroll region; Gitlink's nested scrolling is gone, leaving one scroll region on a dense card.