Consolidate duplicated path/PDF helpers into shared leaf modules (#654)

[abhishek-8081]

Closes #654.

Consolidates the helpers that got duplicated across files during the #640/#642 refactors
into two shared leaf modules:

• app/core/paths.py — PROJECT_ROOT, _resolve_project_file, _resolve_target_directory.
• app/core/pdf_utils.py — _pdf_text, _humanize, _count_pdf_widgets, _extract_pdf_fields,
  _FIELD_TYPE_BY_FT.

Both are dependency-free leaves (import only pathlib/pdfrw/fastapi, nothing from
app.services/app.api), which is what dissolves the circular import (template → controller →
file_manipulator → filler → template) that forced the duplication in the first place.
forms.py, form.py, template.py, filler.py, and purge.py now import from these instead of
holding their own copies.

Three things worth flagging (beyond a pure move):

• The three _resolve_project_file copies weren't identical — forms.py/form.py raised
  AppError, template.py raised HTTPException. The exception is swallowed at every observable
  call site, so I standardized on HTTPException (the only choice provably behavior-identical
  everywhere, including the one path where the type could theoretically surface).
• _count_pdf_widgets depends on _extract_pdf_fields, which needs PROJECT_ROOT — so those
  came along too (pdf_utils reaches PROJECT_ROOT via app.core.paths, an app.core→app.core
  ref, not reopening the cycle).
• purge.py had its own PROJECT_ROOT copy (a fourth site) — consolidated too;
  _safe_delete_file (different logic) left untouched.

Behavior-preserving — 155 tests pass with only monkeypatch-location changes (the 6
PROJECT_ROOT patches now target app.core.paths.PROJECT_ROOT via module-attribute access, so
one patch covers all callers). ruff check app/ clean. Each helper is now defined in exactly
one place.