feature: add mkdocs-material documentation site with CI deployment

Generate comprehensive documentation for all 173 source units and
integrate mkdocs-material into the build toolchain. Documentation
covers kernel core, x86 architecture, drivers, applications, and
all subsystems with a tabbed navigation structure.

- Add mkdocs.yml with material theme, mermaid support, and tabbed nav
- Add doc/index.md landing page
- Generate doc/src/ markdown for every Pascal unit in src/
- Reorganize planning docs into doc/planning/
- Add compat, lvglh, and toolchain doc sections
- Update Dockerfile to install python3 and mkdocs-material (pinned <2)
- Replace pasdoc-based compile_docs.sh with mkdocs build --strict
- Add compile_docs.sh to the main build pipeline in compile.sh
- Add deploy-docs step in .drone.yml with host volume mount to nginx
- Add site/.gitkeep for build output directory

doc/src/app/app.filepicker.md

@@ -0,0 +1,60 @@
+# app.filepicker
+
+Reusable LVGL file picker dialog for open and save operations.
+
+## Overview
+
+`app.filepicker` provides a modal file picker widget that can be embedded by any application needing VFS file selection. It presents a two-pane layout with a bookmarks sidebar and a scrollable file list. Directories and files are displayed separately, both sorted alphabetically. The caller supplies a callback that receives the selected path as a heap-allocated string (which the caller must `kfree`) or `nil` on cancellation.
+
+The picker is built on a full-screen semi-transparent backdrop that blocks interaction with the application behind it. Destroying the backdrop tears down the entire picker tree in one LVGL call.
+
+## Dependencies
+
+- `core.ds.hashmap`, `memory.heap`
+- `driver.video.lvgl`
+- `driver.storage.types`, `driver.storage.vfs`
+- `core.strings`, `io.syslog`, `debug.tracer`
+- `core.util`, `arch.x86.util`
+
+## Constants
+
+### FP_FILTER_ALL
+`nil` — Pass as the `filter` parameter to show all file types.
+
+## Types
+
+### TPickerCallback
+
+```pascal
+TPickerCallback = procedure(path: pchar; userdata: pointer); cdecl;
+```
+
+Callback invoked when the user confirms a selection or cancels. `path` is a `kalloc`'d absolute path string that the caller must `kfree`, or `nil` if the user cancelled.
+
+### TPicker / PPicker (internal)
+
+State record holding all LVGL widget references, current directory, navigation history (up to 16 entries), callback pointer, and picker mode flags.
+
+## Functions and Procedures
+
+### show_open
+
+```pascal
+procedure show_open(title: pchar; start_dir: pchar; filter: pchar;
+                    cb: TPickerCallback; userdata: pointer);
+```
+
+Creates and displays an open-mode file picker. `title` is shown in the dialog header. `start_dir` is the initial directory. `filter` restricts visible files to a specific extension (e.g. `'.txt'`), or pass `FP_FILTER_ALL` for no restriction. `cb` is called with the chosen path on confirm or `nil` on cancel. `userdata` is passed through to the callback unchanged.
+
+### show_save
+
+```pascal
+procedure show_save(title: pchar; start_dir: pchar; initial_name: pchar;
+                    filter: pchar; cb: TPickerCallback; userdata: pointer);
+```
+
+Creates and displays a save-mode file picker. In addition to the open-mode features, a filename entry bar is shown below the file list. If the user selects an existing file a two-click overwrite confirmation is required before the callback is invoked. `initial_name` pre-fills the filename bar.
+
+## Notes
+
+Navigation history is capped at 16 entries. Entry names stored in LVGL widget `user_data` are `stringCopy`'d and freed by `free_list_items` before any list repopulation or picker close. Bookmark button paths are similarly managed by `free_bkmk_items`. The picker uses a deferred-refresh timer to avoid re-entering LVGL event handlers during directory population.