feat(settings): send-tab-style slider for the rescan start height

The optional "scan from block height" control in the Import Private/Viewing
Key dialog now leads with a slider styled like the send-tab amount bar
(rounded track, accent fill, glass thumb) bounded by the chain tip, with the
numeric field beside it for exact entry and the current chain height
right-aligned above. When the node's height is unknown (state_.sync.blocks == 0,
e.g. not yet synced) the slider is omitted and the field spans the full row —
which also fixes the previous cramped 140px field that left the row half empty.

- New static helpers in app.cpp: formatIntWithCommas() and RenderScanHeightControl()
  (mirrors RenderAmountBar in send_tab.cpp). The height decimal string is the single
  source of truth shared by slider + field; the slider reserves/queries its
  InvisibleButton before drawing so a drag has no one-frame lag; the bar height is
  ImGui::GetFrameHeight() so it aligns with the numeric field.
- Import handler clamps the parsed start height to the chain tip.
- i18n: added import_scan_tip ("current height") in en + 8 langs.

Reviewed via an adversarial multi-agent pass (slider-math / state-sync /
dpi-layout / consistency); the one confirmed finding (bar/field height mismatch)
is fixed here.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

res/lang/fr.json

@@ -446,6 +446,7 @@
     "import_private_key": "Importer une clé privée...",
     "import_scan_hint": "0 = réanalyser depuis le début",
     "import_scan_label": "Analyser à partir de la hauteur de bloc (facultatif)",
+    "import_scan_tip": "hauteur actuelle",
     "import_scan_transparent": "Les clés transparentes réanalysent toujours entièrement",
     "import_viewkey_field": "Clé de visualisation",
     "import_viewkey_note": "Lecture seule : une clé de visualisation révèle le solde et les transactions d'une adresse, mais ne peut pas dépenser ses fonds.",