feat(home): multi-file upload + per-file analysis, drop tool grid

Home is now upload + analysis only. The page accepts multiple files in
one go, analyzes each independently, and renders findings grouped by
filename in bordered containers. The 3-section tool-card grid is gone —
discovery happens via the sidebar now.

Mechanics:
- file_uploader uses accept_multiple_files=True. Each file's findings
  cache in session_state["home_findings_by_file"] keyed by filename so
  removing a file via Streamlit's "x" button drops its findings too,
  and re-clicking Run only re-analyzes pending files.
- The first uploaded file is mirrored into the singular
  home_uploaded_{name,bytes,size} keys so tool pages continue to pick
  up an "active" upload through pickup_or_upload — no tool-page changes.
- New i18n keys: upload.intro_multi, upload.uploader_label_multi,
  upload.clear_results, upload.empty_state. upload.heading text is
  updated to "Upload one or more files to start" (EN + ES).

Dropped tests pinning the tool grid:
- TestHomeToolGridLocalization (test_chrome.py)
- test_home_tool_card_uses_es_name (test_smoke.py)
- TestLiteHomeGridBadges (test_lite_tier.py — locked-card lock-badge
  assertions; locking is still enforced per-tool-page via
  require_feature_or_render_upgrade)

2009 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/i18n/packs/en.json

@@ -15,7 +15,7 @@
     "coming_soon": "Coming Soon"
   },
   "upload": {
-    "heading": "📤 Upload a file to start",
+    "heading": "📤 Upload one or more files to start",
     "intro": "Optional: scan an uploaded file for data quality issues and see which tools can fix each one. Skip if you already know what you need.",
     "limits": "**Up to 1.5 GB.** Formats: CSV, TSV, XLSX, XLS. Delimiters auto-detected: comma, tab, semicolon, pipe. Encodings auto-detected: UTF-8 (with/without BOM), UTF-16, cp1252, Latin-1/9, cp1250, ISO-8859-2, cp1251, KOI8-R, Mac Roman, Shift_JIS, GB18030, Big5, EUC-KR — and override on the Review page.",
     "uploader_label": "Upload CSV or Excel",
@@ -27,7 +27,11 @@
     "using_session_file": "Using **{name}** from the upload screen.",
     "use_different_file": "Use a different file",
     "switch_back": "Switch back to upload-screen file",
-    "pickup_caption": "Up to 1.5 GB. Delimiters auto-detected: comma, tab, semicolon, pipe. Encoding auto-detected (UTF-8 / UTF-16 / cp1252 / Latin-1 family / cp1250 / cp1251 / KOI8-R / Mac Roman / Shift_JIS / GB18030 / Big5 / EUC-KR), with override on the Review page."
+    "pickup_caption": "Up to 1.5 GB. Delimiters auto-detected: comma, tab, semicolon, pipe. Encoding auto-detected (UTF-8 / UTF-16 / cp1252 / Latin-1 family / cp1250 / cp1251 / KOI8-R / Mac Roman / Shift_JIS / GB18030 / Big5 / EUC-KR), with override on the Review page.",
+    "intro_multi": "Drop files below. Each one is analyzed locally — no upload anywhere.",
+    "uploader_label_multi": "Upload CSV, TSV, or Excel files",
+    "clear_results": "Clear results",
+    "empty_state": "Upload one or more files to begin. Your data never leaves this computer."
   },
   "findings": {
     "header": "Detected issues",