fix: correct tab rendering and cursor positioning

[s1yle]

Hi Alexey, I saw the TODO regarding tab character support and decided to implement it as a learning exercise.
I understand your policy on new features, so feel free to close this if it's outside the current scope.
I had a lot of fun digging into the atlas and lexer logic!

This PR implements native support for the Tab character (\t) in ded, replacing the previous behavior of inserting 4 spaces.

Instead of hardcoding tab widths in the renderer or lexer, this implementation leverages the existing atlas->metrics system. By assigning a custom horizontal advance (ax) to the \t glyph index, all components (lexer, cursor positioning, and rendering) automatically respect the tab width without additional conditional logic.

• src/free_glyph.c – Added initialization of the tab character’s glyph metrics in free_glyph_atlas_init. The tab’s advance width is set to 4× the space character’s advance (matching the existing width assumption in lexer_chop_char).

• src/lexer.c – Restored the condition && l->content[l->cursor] != '\t' in lexer_trim_left, ensuring tabs are not consumed as whitespace and can be properly tokenized as TOKEN_TAB.

• src/lexer.h – TOKEN_TAB defined.

• src/main.c – The Tab key handling (inserts a literal \t). The TODO comments about smarter indentation are left for future work.