Add Markdown parser and web documentation viewer

[Copilot]

Replaces the static PDF manual with a Markdown-based documentation system rendered in-browser using the app's native styling, with support for GFM tables and merged/colspan cells via the || column-span syntax.

Changes

Dependency

• Added league/commonmark ^2.8 — CommonMark + GFM extension (pipe tables, strikethrough, task lists, autolinks)

func/markdown.php

• parse_markdown(string $markdown): string — parses Markdown and post-processes HTML to inject Bootstrap classes (table-bordered table-striped, table-dark thead, img-fluid, etc.) so rendered output matches native app styling
• apply_cell_merging(string $html): string — post-processes the rendered HTML via DOMDocument to implement the || column-span syntax: GFM renders each || as an empty <td></td>; this function removes those empty cells and applies colspan to the preceding cell
• html_input is set to 'strip' — raw HTML in Markdown files is stripped for security; merged tables must use the || syntax

docs/ — documentation viewer

• docs/index.php — authenticated viewer; dynamically lists all .md files from docs/; file selection uses basename() + in_array() whitelist to block directory traversal
• docs/manual_utilizador.md — user manual ported to Markdown (replaces the PDF)

assets/docs.css

• Responsive two-column layout (sidebar + content area) built entirely on theme.css CSS variables — light/dark mode works automatically

Navigation updates

• index.php — "Manual de Utilizador (PDF)" button replaced with "Documentação" → /docs/; nav link added
• admin/index.php — "Documentação" link added to admin navbar

Merged/colspan table support

Merged cells are supported using the || column-span syntax directly in Markdown tables. Each || absorbs one additional column into the preceding cell's colspan:

| Column 1 | Column 2 | Column 3 |
| -------- | -------- | -------- |
| United with Column 2 || Column 3 |
| Single Line |||

• Row 1: "United with Column 2" renders as <td colspan="2">, "Column 3" is in column 3
• Row 2: "Single Line" renders as <td colspan="3">

Inline Markdown within cells (**bold**, *italic*, `code`) is fully preserved since merging happens after CommonMark renders the cell content.

Adding new documentation requires only dropping a .md file into docs/ — it appears in the sidebar automatically.

[marpisco]

@copilot
No, table support must be done via markdown extended table syntax and not via a regular HTML table.

For example

Column 1	Column 2	Column 3
United with Column 2		Column 3
Single Line

Do you understand?

[Copilot]

@copilot
No, table support must be done via markdown extended table syntax and not via a regular HTML table.

...

Done in commit 9204429.

The || column-span syntax is now implemented via apply_cell_merging() in func/markdown.php. GFM already parses || as an empty cell — the post-processor then uses DOMDocument to remove those empty <td></td> elements and applies colspan to the preceding cell accordingly:

| Column 1 | Column 2 | Column 3 |
| -------- | -------- | -------- |
| United with Column 2 || Column 3 |
| Single Line |||

Produces:

• Row 1: <td colspan="2">United with Column 2</td><td>Column 3</td>
• Row 2: <td colspan="3">Single Line</td>

Inline Markdown within cells (**bold**, *italic*, `code`) is fully preserved since the merging happens after CommonMark renders the cells. Raw HTML passthrough has been removed (html_input changed to 'strip'), so merged tables must use the || syntax.

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