Improve the graph help dialog (issue #7)

[iTerminate]

Summary

• Splits the fullscreen graph screen's dual-purpose keys (left/right/shift+left/shift+right/home/end/enter/escape/q) into mode-accurate twin bindings, so the Footer and help page always describe what a key does right now instead of always showing the paging meaning while in inspect mode.
• Replaces the graph screen's app-key denylist with an explicit allowlist, fixing several leaked keys (n/N search, L, u/ctrl+r) the issue called out, while keeping the ones that still work (d, D, s, T, quit).
• Groups the Graph and Dashboard help pages into labeled sections (Window / Inspect mode / Lines & Colors / Saving / Other / From anywhere, and Use mode / Edit mode / Both modes respectively) so both modes' bindings are visible together.
• Filters meaningless Textual DataTable rows (Cursor left/right, Select, bare Home/End) off the live Main help page.
• Wires ? into Light Control, Media Player, Weather Forecast and Config screens, which previously either didn't respond (modal screens) or silently fell back to the unrelated Main page.

Test plan

• Targeted test batches after each change (test_graph.py, test_graph_cursor_stride.py, test_binary_graph.py, test_climate_graph.py, test_help.py, test_dashboard_*, test_device_tree.py, test_light_control.py, test_media_player_control.py, test_config_screen_*.py, test_entity_control.py)
• uv run ruff check .
• uv run pyright
• Full pytest suite (1083 passed)

Refs #7

https://claude.ai/code/session_01KSut7AwCYk11MxgbVaxwc9