Add `$month_from_text` built-in variable

[michaelbeutler]

Add a built-in variable $month_from_text that detects month names in the document text and resolves to a 2-digit month number. Supports English and German month names.

Detected Text	Resolves To
January / Januar	01
February / Februar	02
March / März	03
April	04
May / Mai	05
June / Juni	06
July / Juli	07
August	08
September	09
October / Oktober	10
November	11
December / Dezember	12

If no month name is found, resolves to empty string (or a configurable default).

Example Usage

output:
  filename: "Test_AG_$y_$month_from_text"

Implementation

• File: crates/paporg/src/config/variables.rs
• Add a new function detect_month_from_text(text: &str) -> Option<String> that scans for month names (case-insensitive)
• Return the first match found (scanning left-to-right)
• Add to builtin variables in get_builtin_variables() — requires access to the document text, so the function signature may need to accept text as parameter
• If no month detected, insert empty string or omit the variable (letting the default mechanism handle it)

Acceptance Criteria

• Detects English month names (January–December)
• Detects German month names (Januar, Februar, März, etc.)
• Case-insensitive detection
• Returns 2-digit zero-padded number
• First occurrence wins when multiple months appear
• Resolves to empty string when no month found
• Covered by unit tests for all 12 months in both languages