fix: remove dead code, ship Fill Forms as a live tool, harden form filling

[Karna14314]

Problem Statement

This PR addresses the dead code audit findings and fixes the orphaned "Fill Forms" feature per the provided specification.

• The HomeScreen.kt was 100% orphaned.
• The "Fill Forms" feature was fully built out but orphaned from the tool list in the live ToolsScreen.kt.
• The "Fill Forms" domain logic lacked defensive error handling, localization, proper validations, and unit tests.

Changes Made

Part A - Dead Code Cleanup

• Verified and deleted HomeScreen.kt: A full grep sweep confirmed zero references to HomeScreen(, ToolCategory, or its localized titles anywhere in the active navigation tree (AppNavigation.kt and Screen.kt).
• Retained PdfRedactor.kt: After confirming zero references in the ui/ layer or README, I've left it intact for human review, as it is a fully implemented domain class under domain/operations/ and could be a parked/future feature, not just a duplicate view like HomeScreen.kt. (I have not modified it).
• Sweep Note: A quick check of ui/screens/ revealed LanguageSelectionScreen, MultiOutputResultScreen and PdfViewerViewModel are unreferenced in AppNavigation.kt. However, this is expected behavior for inner components/view models, so they have been left untouched.

Part B - Wired Fill Forms

• Restored Access: Added the fill_forms tool entry to ToolsScreen.kt under the ToolSection.SECURITY block using the existing string keys tool_fill_forms / desc_fill_forms and the Icons.Default.EditNote icon. Confirmed routing maps correctly (Screen.FillForms).

Part C - Fill Forms Hardening

• Localization: Extracted 10 hardcoded English strings into strings.xml and wired them up in FillFormsScreen.kt using stringResource().
• Validation Gap: Added an explicit UI state for state.fillableFields == 0 (e.g., when a form has fields but all of them are marked read-only).
• Defensive Parsing: Wrapped the PDFBox field conversion logic in a try/catch inside PdfFormFiller.kt, continuing the processing instead of crashing the analyzer.
• Signature Fields: Explicitly typed PDSignatureField to ensure they default to an UnknownField flagged as "Signature", skipping crashes.
• Testing: Added robust PdfFormFillerTest.kt unit tests verifying the fallback logic against a real, mock PDDocument structure (eschewing Robolectric PDFBox asset bugs). Added basic state transition coverage to FillFormsScreenTest.kt.

All tests pass on all 3 flavors (fdroid, opensource, and playstore). No new test regressions were introduced.

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PR created automatically by Jules for task 17965486998119656232 started by @Karna14314

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