Qt-removal R7.5f: remove stale phase promises from user-visible text

[dovvnloading]

Final R7.5 parity sub-increment, closing the phase (a–g). Previous: #147, #148/#149/#150, #151, #152/#153, #154.

Problem

Several disabled controls and one backend notification told users a capability would arrive in an internal project phase (R3/R4/R5/R6) that had already shipped and closed — the app promised "coming in R4" when R4 was long done and the feature still wasn't there.

Scope was derived by grepping the codebase rather than from the plan's "4 R4 placeholders" shorthand, which undercounted it. Eight strings across six files, including a stale R6 promise on DocumentNodeView's Export item and a backend notification in plugins.py's executePlugin fallthrough — neither of which an "R4" search would have found.

Change

Phase-free replacements:

Location	New text
ChatNodeView — Key Takeaway, Explainer Note	"AI note generation isn't available yet"
DocumentNodeView — Export	"Document export isn't available yet"
AppBar — provider mode select	"Switching provider modes isn't available yet"
Composer — attach	"Attachments aren't available yet"
Composer — model/provider	"Model selection isn't available here yet - configure models in Settings"
plugins.py — executePlugin fallthrough	node creation "isn't available yet"
HtmlNodeView — popout	keeps the real security reason, drops the internal plan-doc citation
SceneCanvas — placeholder node body	"Placeholder node"

Every new claim was checked against the code before being written, to avoid replacing one false statement with another. Model selection is genuinely reachable in Settings for all three providers (setOllamaModelAssignment, saveApiConfiguration/loadApiModels, setLlamaCppChatModelPath), so the one string that points somewhere is accurate. Provider-mode switching exists nowhere — set_current_mode is only called by bootstrap_provider_state at startup — so that string deliberately offers no workaround.

Five coupled test assertions updated to match.

Test plan

• 885 backend / 1208 frontend tests pass, 0 lint errors, typecheck/build clean
• Burn-down gate unchanged at 152/84/68 (text-only, no Qt files touched)
• Adversarial review verified each new string against the code — confirmed all five remaining deferrals are genuinely unimplemented (no takeaway/explainer backend symbols; capabilities.attachments still False; downloadTextFile imported only by ChatNodeView/CodeNodeView) and that the HtmlNodeView rewording matches the actual sandbox posture (sandbox="allow-scripts", no allow-same-origin)
• Review caught one miss, now fixed: SceneCanvas's PlaceholderNodeView rendered "placeholder — real nodes land in R3" as body text — the most visible instance, reachable by double-clicking empty canvas and as the fallback for any unrecognized node kind. The initial sweep covered title attributes and quoted strings but not JSX text content.
• Final sweep confirms zero user-visible phase references remain in web_ui/src/app/ or backend/; remaining hits are code comments, docstrings, and test-failure messages (internal history, out of scope)